The selection of a promoter in the genetic engineering of a heterologous gene is often a critical factor in obtaining expression of the desired gene. It is the nucleotide sequence of the promoter at the 5' end of a structural gene which is either directly or indirectly recognized and bound by a DNA-dependent RNA polymerase during the initiation of transcription of the gene. It is generally recognized by those skilled in the art, that the localization of gene expression is desirable in a tissue-specific manner.
Expression of tissue-specific plant genes has been identified genetically and by their differential expression in various organs, for example: embryo-specific gene expression of seed storage proteins including, glycinin, legumins, zeins, and glutenin; fruit ripening gene expression including polygalacturonase; and differential expression of root nodulation genes in leguminous plants including nodulin. It would be desirable to regulate tissue specific heterologous gene expression with a promoter which was preferential to the specific tissue where expression was required. The ability to control levels of gene expression is very important and expression of the protein product may impart increased agronomic, horticultural and/or pesticidal characteristics to a given crop plant.